Regarding mobile communication systems the essential operating frequencies are located in a frequency domain, which extends from hundreds of megahertz to several gigahertz, and in microwave links between the stationary parts of the network, even to tens of gigahertz. The filters which are used at these frequencies are generally based on resonators comprising an inner conductor surrounded by an outer conductor. The resonators are divided into different types, usually on the basis of structural details. Known resonator types are for instance the helix, the coaxial, the microstrip and the dielectric resonators; the filters are correspondingly called helix, coaxial, microstrip and dielectric filters. The filter resonators are also called filter stages or filter circuits.
FIG. 1 is a schematic cross-section of a known coaxial filter having three resonators. The inner conductors 101 of the resonators are fastened at one end (in the figure the lower end) to a printed circuit board 102 so that the printed circuit board has a hole 103 for each inner conductor, into which hole the lower end of the inner conductor is pushed. On the bottom surface of the printed circuit board 102 there is a substantially continuous ground plane 104, which after the assembly is connected in an electrically conducting manner (for instance by solder) to the lower end of each inner conductor. The top surface of the printed circuit 102 board comprises electrically conducting patterns 105, which arrange the connections from a filter input port to the resonator which is closest to the input port, and correspondingly to the filter's output port from the resonator which is closest to the output port. The patterns 105 can further have an effect on the electromagnetic couplings between the resonators. The inner conductors are enclosed in an electrically conducting box structure 106 which forms the outer conductor of each resonator and which at the same time acts as the mechanical outer cover of the filter. At the edges it is connected to the ground plane 104 on the bottom surface of the printed circuit board 102. The input and output ports of the filter are isolated from the electrically conducting outer cover.
A relatively large number of components is typical for the prior art filter structures. This causes inconvenience in the assembly because the large number of separate components increases manufacturing costs and tends to cause dimensioning variations in the final products. Further it is typical to the prior art filter structures comprising several components that they have intermodulation problems, which means a non-linear mixing of two or more signals and the non-harmonic frequencies resulting from the mixing. The connection surfaces of the separate components connected to each other form a significant source of intermodulation, particularly when the signals have a high power level. The rusty bolt effect means a phenomenon where contact surfaces, not well matched to each other, an insufficient tightening torque, oxidation, corrosion, impurities on the contact surfaces, or some other factor, results in that the electrically conducting continuous surfaces in the junction between two metal bodies are not tightly abutting each other. Then there exists a substantial uncontrolled resistance and/or capacitance between them, which causes non-linear current-voltage effects in the electric current passing through the junction. The non-harmonic frequencies generated by the intermodulation can be extremely harmful, for instance if they happen to overlap a useful signal.